1. Field of the Invention
The present invention relates to a plasma display device having a three-electrode AC (Alternating Current) type of plasma display panel and a method for driving the plasma display device.
The present application claims priority of Japanese Patent Application No. 2003-307915 filed on Aug. 29, 2003, which is hereby incorporated by reference.
2. Description of the Related Art
A plasma display panel (hereinafter may be referred to simply as a “PDP”) has, in general, many advantages in that it can be made thin, display on a large screen is made possible with comparative ease, it can provide a wide viewing angle, it can give a quick response, and a like. Therefore, in recent years, the PDP is being widely and increasingly used, as a flat display panel, for wall-hung TVs, public information boards, or a like. The PDP is classified, depending on its operating method, into two types, one being a DC (Direct Current) discharge-type PDP whose electrodes are exposed in a discharge space (discharge gas) and which is operated in a direct-current discharge state and another being an AC (Alternating Current) discharge-type PDP whose electrodes are coated with a dielectric layer and are not exposed directly in a discharge gas and which is operated in an alternating-current discharge state. In the DC-type PDP, while a voltage is being applied, discharge continues to occur. In the AC-type PDP, discharge is sustained by reversing a polarity of a voltage to be applied. The AC-type PDP is also classified, depending on the number of electrodes in one cell, into two types, one being a two-electrode type AC-type PDP and another being a three-electrode AC-type PDP.
Configurations and driving method of the conventional three-electrode AC-type PDP are described below. FIG. 17 is a cross-sectional view illustrating configurations of one cell in the conventional three-electrode AC-type PDP. FIG. 18 is a plan view illustrating configurations of the conventional three-electrode AC-type PDP. FIG. 19 is a diagram showing driving waveforms of pulses to be applied in the conventional three-electrode AC-type PDP.
The conventional three-electrode AC-type PDP, as shown in FIG. 17, has a front substrate 20 and a rear substrate 21, both facing each other, two or more scanning electrodes 22, two or more sustaining electrodes 23, and two or more data electrodes 29, all being placed between the front substrate 20 and the rear substrate 21, and display cells being arranged in a matrix form and each being placed in a portion of intersection among each of the scanning electrodes 22, each of the sustaining electrodes 23, and each of the data electrodes 29.
The front substrate 20 is made up of a glass substrate or a like, on which each of the scanning electrodes 22 and each of the sustaining electrodes 23 is placed at a specified interval between them. On each of the scanning electrodes 22 and sustaining electrode 23 is formed a metal trace electrode 32 to lower wiring resistance. On the scanning electrodes 22, sustaining electrodes 23, and metal trace electrodes 32 is formed a transparent dielectric layer 24 and, further, in order to protect the transparent dielectric layer 24 from discharge, a protecting layer 25 made of magnesium oxide (MgO) or a like is formed on the transparent dielectric layer 24. The rear substrate 21 is made up of a glass substrate, or a like, on which each of the data electrodes 29 is formed in a manner to be orthogonal to each of the scanning electrodes 22 and sustaining electrodes 23. On the data electrodes 29 are formed a white dielectric layer 28 and a phosphor layer 27. Between the front substrate 20 and rear substrate 21 are formed parallel-cross shaped ribs 33 in a manner to surround each cell. Each of the ribs 33 plays a role of securing a discharge space 26 and of partitioning pixels. Each discharge space 26 is filled with a mixed gas made of, as discharge gas, helium (He), neon (Ne), xenon (Xe) or a like in a hermetically sealed manner.
In the conventional three-electrode AC-type PDP, as shown in FIG. 18, display cells are arranged in a matrix form, each being formed in a portion of intersection among each electrode Si (i=1 to m) making up the scanning electrode 22, each electrode Ci (i=1 to m) making up the sustaining electrode 23, and each electrode Dj (j=1 to n) making up the data electrode 29.
Next, a method for driving a PDP is described. Presently, the method for driving the PDP being in a mainstream is an ADS (Address and Display Separation) method in which operations are performed in its scanning period and sustaining period in a separated manner. The ADS method is explained by referring to FIG. 19. FIG. 19 shows one example of driving waveforms of pulses applied during one sub-field (called simply as an “SF” in drawings) 5 employed in the conventional three-electrode AC-type PDP. One sub-field 5 includes three periods including an initializing period 2, a scanning period 3 and a sustaining period 4.
First, operations in the initializing period 2 are described. As shown in FIG. 19, before the initializing period 2, a sustaining period 1 in a previous sub-field exists and an amount of wall charges to be formed, which are charges accumulated by discharge on a dielectric layer on each electrode in a cell, varies depending on whether or not sustaining discharge has occurred during the sustaining period 1 in the previous sub-field. If writing on a following line is done in a state in which wall charges formed by the discharge occurred during the sustaining period 1 in the previous sub-field are still left, due to influences caused by the left wall charges being different depending on a lighting state of a cell in the sustaining period 1, occurrence of smooth writing discharge is made difficult, thus causing erroneous writing. One of roles of operations to be performed during the initializing period 2 in the sub-field 5 is to reset, for initialization, a state of accumulated wall charges which vary depending on a lighting state of a cell during the sustaining period 1 in the previous sub-field and which are charges formed by discharge on a dielectric layer in the cell.
The setting for initialization is made mainly during a sustaining erasing period 8 in the initializing period 2, as shown in FIG. 19. During the sustaining erasing period 8, only when sustaining discharge occurs during the sustaining period 1 in the previous sub-field, feeble discharge occurs between each of the scanning electrodes 22 and sustaining electrodes 23 and between each of the scanning electrodes 22 and data electrodes 29. Unlike in the case of intense discharge that occurs at a dash by application of a pulse having a rectangular waveform which reverses, at a stroke, a polarity of a wall charge formed on the electrodes, feeble discharge occurs in a sustained manner by a gradual change in a voltage at each of the scanning electrodes 22 according to a ramp waveform of an applied pulse during the sustaining erasing period 8, which produces a little change in wall charges formed on the electrode by discharge.
On the other hand, operations during the initializing period 2 have additional roles of providing a priming effect by which discharge is made easy when data is written in a one-pass scanning manner according to data to be displayed and of putting a state of wall charges into a state in which writing discharge occurs in an optimized manner. These roles are realized mainly during a priming period 9 and during a wall charge adjusting period 10. During the priming period 9, feeble discharge occurs regardless of whether or not sustaining discharge occurred during the sustaining period 1 in the previous sub-field and this discharge causes priming particles in cell space which serves to induce a state in which writing discharge is likely to occur easily. Moreover, during the priming period 9, a potential of each of the scanning electrodes 22 increases gradually in a manner to have positive polarity relative to a potential of each of the data electrodes 29 and, as a result, negative wall charges increase on each of the scanning electrodes 22 and positive wall charges increase on each of the data electrodes 29. Production of priming particles and increases in wall charges as described above serve to cause writing discharge to occur easily and, in the case in which a cell has continued to be not lit for a long time in particular, since priming particles and wall charges tend to decrease, the above production of priming particles and the increases in wall charges work to compensate for these decreases.
In the wall charge adjusting period 10, amounts of wall charges formed on each of the electrodes during the priming period 9 are adjusted so that a display panel can operate in a proper manner. Also, in the wall charge adjusting period 10, as in the case of the initializing period 2, feeble discharge occurs between each of the scanning electrodes 22 and each of the sustaining electrodes 23 and between each of the scanning electrodes 22 and each of the data electrodes 29. Moreover, in the wall charge adjusting period 10, since a data electrode potential is fixed to be at a ground potential and a scanning electrode potential lowers gradually according to the ramp waveform of a pulse, the ultimate potential of the scanning electrode potential becomes almost the equal to a potential of a scanning pulse 6. In a final stage of the feeble discharge, the potential between each of the scanning electrodes 22 and each of the data electrodes 29 is put in a state in which amounts of the wall charges are changed by discharge to a level at which discharge is likely not to occur until immediately before an end of the scanning period 3. That is, in the wall charge adjusting period 10, between each of the scanning electrodes 22 and each of the data electrodes 29, a state occurs in which wall charges are reduced to a level at which discharge does not occur unless a data pulse 7 is applied at the same time when the scanning pulse 6 is applied.
On the other hand, wall charges are in a state in which, if a positive pulse is applied even a little to each of the data electrodes 29, discharge occurs and, therefore, writing discharge occurs at a low data pulse voltage. However, since time is required before discharge occurs after application of a voltage in actual operations, in order for discharge to occur during a period for which such a pulse having a short wavelength as the scanning pulse 6 is being applied, some data pulse voltage is needed. In the initializing period 2, as described above, a cell state being optimized to resetting for initialization of wall charges and to occurrence of writing discharge is realized.
Next, operations during the scanning period 3 are explained. The scanning period 3 is a period during which a state of wall charges is sequentially changed for each of the scanning electrodes 22 according to video signals in a manner to correspond to occurrence or non-occurrence of writing discharge to write video information into a cell. During the scanning period 3, a scanning pulse 6 is applied sequentially to each electrode (S1 to Sm) making up the scanning electrode 22. With timing with which the scanning pulse 6 is applied, a data pulse 7 is applied, in a manner to correspond to a display pattern, to each electrode (D1 to Dn) making up the data electrode 29. A sloped line in the data pulse 7 in FIG. 19 represents that the data pulse 7 is applied or not applied according to video signals.
Occurrence or non-occurrence of writing discharge is determined in a way described below. While the data pulse 7 is being applied, a potential between each of the scanning electrodes 22 and each of the data electrodes 29 is a potential difference “Vd”. At this time point, as described above, a negative charge is formed on each of the scanning electrodes 22 and a positive charge is formed on each of the data electrodes 29. Since voltages of wall charges applied to a dielectric layer by these wall charges are superimposed on the potential difference between each of the scanning electrodes 22 and each of the data electrodes 29, a high voltage is generated in the discharge space 26 between each of the scanning electrodes 22 and each of the data electrodes 29 and, as a result, writing discharge occurs between each of the scanning electrodes 22 and each of the data electrodes 29. At this time point, since a big potential difference between each of the scanning electrodes 22 and each of the sustaining electrodes 23 is also produced, when the writing discharge occurs between each of the scanning electrodes 22 and each of the data electrodes 29, surface discharge is induced between each of the scanning electrodes 22 and sustaining electrodes 23 and, therefore, positive wall charges are accumulated on each of the scanning electrodes 22 and negative wall charges are accumulated on each of the sustaining electrodes 23.
On the other hand, in cells to which no data pulse 7 is fed, since a difference of a potential to be applied in the discharge space 26 between each of the scanning electrodes 22 and each of the data electrodes 29 does not exceed a discharge starting voltage, no discharge occurs and the state of wall charges remain unchanged. Thus, two types of states of wall charges can be obtained depending on whether the data pulse 7 is applied or not.
After the application of the scanning pulse 6 has been completed to all lines, operations in the sustaining period 4 start. A sustaining pulse is alternately applied to all the scanning electrodes 22 and all the sustaining electrodes 23. Since a voltage “Vs” of the sustaining pulse is adjusted so as to be almost the same as a wall voltage occurring in the vicinity of a discharge gap 34 between each of the scanning electrodes 22 and each of the sustaining electrodes 23 in cells in which writing discharge did not occur, only the voltage “Vs” being a potential difference between a voltage at each of the scanning electrodes 22 and a voltage at each of the sustaining electrodes 23 is applied in the discharge gap 34 between each of the scanning electrodes 22 and each of the sustaining electrodes 23 and, therefore, discharge (the discharge occurring between each of the scanning electrodes 22 and each of the sustaining electrodes 23 is called a “surface discharge”) does not occur between each of the scanning electrodes 22 and each of the sustaining electrodes 23.
On the other hand, in cells in which writing discharge has occurred, since a positive wall charge is formed on each of the scanning electrodes 22 and a negative wall charge is formed on each of the sustaining electrodes 23 and since the positive and negative wall charges are superimposed on a first voltage of the positive sustaining pulse (called as a “first sustaining pulse”) to be applied to each of the scanning electrodes 22 and, since a voltage exceeding a discharge starting voltage is applied in the discharge gap 34, sustaining discharge occurs. This sustaining discharge causes negative wall charges to be accumulated on each of the scanning electrodes 22 and positive wall charges to be accumulated on each of the sustaining electrodes 22.
A next sustaining pulse (called a “second sustaining pulse”) is applied to each of the sustaining electrodes 23 and wall charges described above are superimposed on a voltage of the second sustaining pulse and, therefore, also sustaining discharge occurs here, thus causing wall charges having a polarity being reverse to that of the first sustaining pulse to be accumulated on both each of the scanning electrodes 22 and each of the sustaining electrodes 23. Thereafter, discharge occurs by the same operations as above in a sustained manner. That is, a potential produced by wall charges formed by “x-th” time sustaining discharge is superimposed on a voltage of a next “x+1st” time sustaining pulse and the sustaining discharge continues to occur. Light-emitting luminance is determined by the times of sustaining occurrences of this sustaining discharge.
A total period including the initializing period 2, scanning period 3, and sustaining period 4 described above is called a “sub-field” (SF)”. When a gray scale is displayed by a display device, one field during which one screen of image information is displayed includes two or more sub-fields. The gray-scale display can be realized by changing the number of the sustaining pulses during each sub-field to cause lighting or non-lighting of a cell during each of the sub-fields.
In the method for driving the conventional AC-type PDP, even if a pulse having the same driving waveform is applied, since intense and/or expansion, or a like of discharge are changed according to a change in a state of a cell in the PDP, an amount of wall charges to be formed in a cell and/or an amount of space charges vary. In particular, if an amount of wall charges and/or an amount of space charges are changed in the initializing period, a writing discharge state during the scanning period thereafter varies which, therefore, causes erroneous non-lighting or erroneous lighting. Such the change of a state in the cell occurs mainly in a manner to correspond to a temperature of a panel or a total driving time during which the panel was operated until then.
As a measure against a writing discharge failure caused by such the change of a state in the cell, a driving method is disclosed in Japanese Patent Application Laid-open No. Hei 9-6283 ([0210] to [0220]) in which a driving waveform is switched in a manner to correspond to a panel temperature. In the sixth embodiment of the above disclosed example, a counter measure against the writing discharge failure caused by the panel temperature is taken by switching the driving waveform during the initializing period (this is called a “reset period” in the example) in a manner to correspond to the panel temperature.
In addition to this, as a measure to perform a more reliable initializing process while operations are performed at a high temperature of a panel, another driving method is disclosed in Japanese Patent Application No. 2002-207449 ([0022]) in which an initializing period (this is called a “blank period+reset period” in the disclosed example) is made longer while operations are performed at a high panel temperature and in which it is described that, by making long the blank period making up the initializing period, space charges decrease, thus enabling occurrence of erroneous discharge to be avoided.
In the case of the above-described conventional method by which a measure against a writing discharge failure caused by panel temperatures by switching a driving waveform during the initializing period in a manner to correspond to the panel temperatures, driving during the initializing period is performed by self-erasing discharge using a rectangular waveform and not using a ramp waveform as shown in FIG. 19. The self-erasing discharge is intense discharge and, if the initializing operation is performed by such the discharge, wall charges cannot be controlled in a delicate manner. This presents a problem in that optimized initialization by using the conventional method becomes difficult.
Also, in the conventional method by which, by making the operating time longer during the initializing period at a high panel temperature and by making longer the blank period making up the initializing period in particular, space charges are made to be decreased to avoid the occurrence of erroneous discharge, however, this method presents a problem in that, to avoid erroneous discharge, the control on space charge is insufficient and wall charges have to be controlled according to a change of a state of a cell.